1. Field of the Invention
Preparation of resorcinol and substituted resorcinols by catalytic dehydrogenation of certain 1,3-cyclic diones is well known. Of particular interest herein are process improvements in the preparation of resorcinol and substituted resorcinols made by dehydrogenation of 1,3-cyclic diones derived by the vapor-phase cyclization of delta-keto carboxylic acid esters.
2. State of the Art
U.S. Pat. No. 4,072,660 to Muller et al. describes the preparation of resorcinol and substituted resorcinols by cyclization of 4-oxocarboxylic acid esters to cyclohexane-1,3-dione type intermediates, followed by dehydrogenation of the intermediates to the corresponding resorcinol or substituted resorcinols. The cyclohexane-1,3-dione intermediate is prepared by a first-step liquid-phase cyclization of a 4-oxocarboxylic acid alkyl ester or a delta-enollactone in a glycol ether solvent in the presence of a stoichiometric amount of a strong base such as sodium methylate to provide a sodium salt of the cyclohexanedione, and a second step of acidification of the sodium salt to form the cyclohexanedione intermediate. Dehydrogenation of the intermediate is accomplished in the same glycol ether solvent in the presence of a supported noble metal catalyst, such as a palladium-on-carbon catalyst. The Muller '660 preparation has the disadvantages of requiring an expensive solvent for the liquid-phase preparation of the cyclohexanedione intermediate, and of requiring a strong alkali catalyst for the cyclization step followed by the need for a strong acid to spring the cyclohexanedione from the alkali salt of the cyclization product. Residual mineral acid from the cyclohexanedione springing step can inhibit the activity of the noble metal catalyst during the dehydrogenation step. Moreover, the Muller '660 preparation generates significant quantities of effluent, the recovery and disposal of which is costly.
In U.S. Pat. No. 4,250,336 to Muller et al., cyclization of a delta-ketocarboxylic acid ester and dehydrogenation of the cyclohexane-1,3-dione to the corresponding resorcinol or substituted resorcinol are accomplished in a one-step gas phase reaction in the presence of hydrogen/nitrogen carrier and a catalyst. The catalyst is typically provided by a two-component mixture of thorium-on-carbon and platinum/chrome oxide-on-alumina. This one-step process is disadvantageous inasmuch as the reaction product contains substantial amounts of various by-products and residual delta-ketocarboxylic acid ester starting material. It is difficult to separate commercially-useable resorcinol or substituted resorcinol from the Muller '336 reaction product mixture, and especially from the delta-ketocarboxylic acid ester starting material and the cyclohexane-1,3-dione intermediate in the reaction product mixture. Moreover, the process requires an expensive catalyst which, once deactivated by use in the one-step cyclization-dehydrogenation reaction, cannot practically be reactivated to the previous degree of activation.
In U.S. Pat. No. 4,160,113 to Muller et al, resorcinol and substituted resorcinols are prepared by catalytic dehydrogenation of cyclohexane-1,3-diones in a liquid phase. The liquid phase is provided by a large variety of dehydrogenation solvents having a wide range of boiling points. Where low-boiling solvents are used, i.e., boiling points below about 160.degree. C., the reaction must be under pressure in order to utilize optimum dehydrogenation temperatures. Thus, a preferred class of dehydrogenation solvents is provided by glycol ethers having boiling points in a range from about 180.degree. C. to about 260.degree. C. Use of many of these Muller '113 high-boiling solvents for dehydrogenation media is disadvantageous inasmuch as the solvent, the resorcinols and other phenolic by-products may have boiling points so similar that separation of desired product from by-products and from the solvent is very difficult.
There remains need, therefore, for a process for making resorcinol or substituted resorcinol in high yields with relatively low amounts of easily separable by-products.